Communication method, server, and wireless distribution system

ABSTRACT

A communication method includes acquiring information relating to a first wireless terminal and a second wireless terminal, deciding priority of data based on information relating to the first wireless terminal and the second wireless terminal, generating a packet including the data regarding which the priority has been decided, acquiring channel-used time, calculating channel-usable time based on the channel-used time, calculating a first channel usage estimated time, calculating a wireless transmission rate to be applied when the wireless base station transmits the packet to the first wireless terminal and the second wireless terminal, based on the priority of the data included in the packet, to where the first channel usage estimated time is within the channel-usable time, and transmitting the packet, and information indicating the wireless transmission rate, to the wireless base station.

BACKGROUND 1. Technical Field

The present disclosure relates to a communication method, a server, anda wireless distribution system.

2. Description of the Related Art

Examples of communication methods used on a wireless network configuredaccording to wireless local area network (LAN) stipulated in IEEE 802.11include unicast, multicast, and broadcast. Unicast is a communicationmethod where packets are exchanged on a one-to-one basis between aparticular wireless base station and a particular wireless terminal,between two particular wireless terminals, or the like. Multicast is acommunication method where at least one or more groups (hereinafterreferred to as “multicast group”) is formed, and packets are transmittedfrom a wireless base station or wireless terminal participating in amulticast group to other wireless base stations or wireless terminalsparticipating in the same multicast group. This communication method isused in cases where communication is performed by Internet Protocol (IP)multicast, for example. Broadcast is a communication method oftransmitting packets without specifying a destination. Thiscommunication method is used when transmitting advertisement beaconframes from a wireless base station to wireless terminals, for example.

Note that reception confirmation by acknowledgement (Ack) frame is notperformed in response to transmission of a frame over the wireless LAN,in the communication methods by multicast and broadcast described above.Accordingly, even if a frame transmission error occurs, frameretransmission over the wireless LAN is not performed automatically.

Next, a transmission method of packets by IP multicast (hereinafterreferred to as “multicast packet”) will be described. Communication isperformed in IP multicast using an IP address for identifying amulticast group, called a multicast IP address (see Japanese Patent No.4386292 and Japanese Unexamined Patent Application Publication No.2007-329614, for example). Specifically, when a server transmits amulticast packet addressed to a multicast group, a multicast IP addresscorresponding to this multicast group is specified as the target addressof the multicast packet.

On the other hand, when a wireless terminal (client) has received amulticast packet transmitted addressed to a multicast group, themulticast packet addressed to the multicast IP address corresponding tothat multicast group is received. Accordingly, the wireless terminaldeclares to the wireless base station on the IP network connected toitself that it will receive the packet addressed to the multicast IPaddress, by notification of the multicast IP address using a joinmessage according to the Internet Group Management Protocol (IGMP) inthe case of IPv4, for example.

When the wireless base station receives the join message, a distributiontree is configured between the wireless base station and router on theIP network such that multicast packets addressed to the multicast IPaddress will be transferred to the wireless terminal. Accordingly, thewireless base station and router on the IP network transfer multicastpackets to the wireless terminal based on the distribution tree that hasbeen configured.

However, there is concern regarding the conventional communicationmethod by multicast that multicast packets may not be able to betransmitted in real-time on the wireless network.

SUMMARY

One non-limiting and exemplary embodiment provides a communicationmethod, server, and wireless distribution system, that can transmitmulticast packets in real-time on a wireless network.

In one general aspect, the techniques disclosed here feature acommunication method of a server that transmits a packet by multicast orbroadcast via a wireless base station to at least one or more wirelessterminals connected to the wireless base station via a wireless networkusing a channel. The method includes: acquiring information relating tothe wireless terminal; deciding priority of data based on informationrelating to the wireless terminal; generating the packet including thedata regarding which the priority has been decided; acquiringchannel-used time that is time over which the channel is used per unittime at the wireless base station; calculating channel-usable time thatis time over which the channel can be used per unit time, for thewireless base station to transmit the packet to the wireless terminal,based on the channel-used time; calculating a first channel usageestimated time that is time estimated to be used for the wireless basestation to use the channel to transmit the packet to the wirelessterminal; calculating a wireless transmission rate to be applied whenthe wireless base station transmits the packet to the wireless terminal,based on the priority of the data included in the packet, to where thefirst channel usage estimated time is within the channel-usable time;and transmitting the packet, and information indicating the wirelesstransmission rate, to the wireless base station.

In a communication method according to an aspect of the presentdisclosure, when transmitting data by multicast or broadcast on awireless network, data can be transmitted in real-time by adaptivelycontrolling wireless transmission rates based on priority of data.

It should be noted that general or specific embodiments may beimplemented as a system, a method, an integrated circuit, a computerprogram, a storage medium such as a computer-readable Compact DiscRead-Only Memory (CD-ROM), or any selective combination of the system,method, integrated circuit, computer program, and storage medium.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the configuration of a wirelessdistribution system according to an embodiment;

FIG. 2 is a block diagram illustrating the configuration of a wirelessbase station according to an embodiment;

FIG. 3 is a block diagram illustrating the configuration of a serveraccording to an embodiment;

FIG. 4 is a diagram illustrating an example of a wireless transmissionrate control management table;

FIG. 5 is a diagram illustrating an example of a wireless terminalmanagement table;

FIG. 6 is a block diagram illustrating the configuration of a wirelessterminal according to an embodiment;

FIG. 7 is a flowchart illustrating processing at a server according toan embodiment;

FIG. 8 is a flowchart illustrating processing at a server according toan embodiment;

FIG. 9 is a flowchart specifically illustrating wireless transmissionrate changing processing in FIG. 8;

FIG. 10A is a diagram for describing an example of wireless transmissionrate changing processing in FIG. 8;

FIG. 10B is a diagram for describing an example of wireless transmissionrate changing processing in FIG. 8; and

FIG. 10C is a diagram for describing an example of wireless transmissionrate changing processing in FIG. 8.

DETAILED DESCRIPTION

Underlying Knowledge Forming Basis of the Present Disclosure

The present inventors have found that the following problems occur withregard to the technology described in the Related Art. First, accesscontrol in a wireless LAN will be described. Autonomous decentralizedcontrol called distributed coordination function (DCF) is performed in awireless LAN based on Carrier Sense Multiple Access/Collision Avoidance(CSMA/CA), using a wireless frequency (hereinafter referred to as“channel”) that has been selected beforehand. Access control using DCFin a wireless distribution system will be described below.

A wireless distribution system has a server, a wireless base station, afirst wireless terminal, and a second wireless terminal. The server isconnected to the wireless base station via a cabled network. Thewireless base station configures a wireless network using a channel. Thefirst wireless terminal and second wireless terminal, which are clients,are connected to the wireless base station via the wireless network.

In a case of transmitting a packet to the server, the first wirelessterminal uses a channel to transmit a frame including the packet to thewireless base station. While the first wireless terminal is transmittingthe frame using this channel at this time, the wireless base station andsecond wireless terminal cannot transmit frames, in order to preventchannel interference from occurring on the wireless network. In a casewhere another wireless network using the same channel as this channel isalso reached by radio waves transmitted by the first wireless terminal,the wireless base station and wireless terminals making up this otherwireless network are not able to transmit frames either, as long as thefirst wireless terminal is transmitting the frame using this channel.

Now, wireless transmission rate in a wireless LAN will be described. Awireless transmission rate is selected from multiple wirelesstransmission rates to transmit frames in a wireless LAN. For example,the IEEE 802.11a standard enables a wireless transmission rate to beselected from the eight wireless transmission rates of 6, 9, 13, 18, 24,36, 48, and 54 Mbps.

Generally, the lower the wireless transmission rate is, the fewer errorsoccur on the wireless network, and the frame can be made to reachfarther away. This improves reliability regarding frame transmission. Onthe other hand, the higher the wireless transmission rate is, the moreinformation can be transmitted in a short time. Accordingly, whentransmitting frames of the same length, the higher the wirelesstransmission rate is, the shorter the time using the channel can bemade.

Next, the wireless transmission rate in a case of transmitting a frameon a wireless network by multicast in the above-described wirelessdistribution system will be described. When the server has transmitted aframe by multicast to the first wireless terminal and second wirelessterminal using IP multicast, the wireless base station transmits theframe received from the server to the first wireless terminal and secondwireless terminal via the wireless network. Generally, in a case wherethe distance between the wireless base station and the second wirelessterminal is longer than the distance between the wireless base stationand the first wireless terminal, the attenuation of radio waves reachingthe second wireless terminal from the wireless base station is greaterthan the attenuation of radio waves reaching the first wireless terminalfrom the wireless base station.

As described above, in a case of transmitting a frame by multicast on awireless network, retransmission of the frame is not automaticallyperformed in a case where a frame transmission error occurs.Accordingly, the slowest wireless transmission rate is selected tobroaden the frame arrival range, in order to secure frame transmissionreliability with regard to multiple wireless terminals in differentradio wave reception states. However, transmitting high-bitrate data inreal-time becomes difficult if the selected wireless transmission rateis slow.

The method disclosed in Japanese Patent No. 4386292 addresses this issuewhen a wireless base station transmits a frame by multicast to multiplewireless terminals, by performing prior registration of the multicast IPaddress to be used and bitrate information of data included in the frameto be transmitted from the server to that multicast IP address. Uponhaving received a frame addressed to the registered multicast IP addressfrom the server, the wireless base station transmits the frame bymulticast to multiple wireless terminals using the wireless transmissionrate suitable for the bitrate of data included in this frame. Themultiple terminal devices each select a bit rate for data to bereceived, in accordance with the radio wave reception state betweenitself and the wireless base station, and transmit an IGMP join messageincluding a multicast IP address corresponding to this data.Accordingly, each of the multiple wireless terminals can receive data inreal-time at a bitrate suited to the radio wave reception state, whilemaintaining reliability of frame transmission.

However, the method disclosed in Japanese Patent No. 4386292 does nottake into consideration the occurrence of channel interference. Forexample, when a wireless terminal starts transmitting a frame, thewireless base station cannot perform frame transmission at all.Accordingly, frames in transmission standby are accumulated at thewireless base station while the wireless terminal is transmitting aframe. This makes transmission of data in real-time difficult for theserver.

In the method disclosed in Japanese Unexamined Patent ApplicationPublication No. 2007-329614, when transmitting a frame by multicast, thewireless base station selects, based on time where a channel is notbeing used (hereinafter referred to as “channel-unused time”) and thebitrate of data transmitted to the multicast IP address, an optimalwireless transmission rate suitable for transmission of a frame to thismulticast IP address. Specifically, the wireless base station calculatesthe channel-unused time per unit time. Further, the wireless basestation selects the wireless transmission rate based on the bitrate ofdata transmitted from the server to the multicast IP address, such thattime of using a channel in a case of having selected a wirelesstransmission rate for transmission of the frame is within thechannel-unused time.

However, the method disclosed in Japanese Unexamined Patent ApplicationPublication No. 2007-329614 does not take into consideration cases wherethe channel-unused time is extremely short. Specifically, when the timeof using a channel in a case where the fastest wireless transmissionrate has been selected for frame transmission exceeds the channel-unusedtime, frames in transmission standby are accumulated at the wirelessbase station. This makes transmission of data in real-time difficult forthe server.

A communication method according to an aspect of the present disclosureis a communication method of a server that transmits a packet bymulticast or broadcast via a wireless base station to at least one ormore wireless terminals connected to the wireless base station via awireless network using a channel. The method includes: acquiringinformation relating to the wireless terminal; deciding priority of databased on information relating to the wireless terminal; generating thepacket including the data regarding which the priority has been decided;acquiring channel-used time that is time over which the channel is usedper unit time at the wireless base station; calculating channel-usabletime that is time over which the channel can be used per unit time, forthe wireless base station to transmit the packet to the wirelessterminal, based on the channel-used time; calculating a first channelusage estimated time that is time estimated to be used for the wirelessbase station to transmit the packet to the wireless terminal;calculating a wireless transmission rate to be applied when the wirelessbase station transmits the packet to the wireless terminal, based on thepriority of the data included in the packet, to where the first channelusage estimated time is within the channel-usable time; and transmittingthe packet, and information indicating the wireless transmission rate,to the wireless base station.

According to this aspect, the server controls the wireless transmissionrate based on the priority of the data included in the packet, so thatthe first channel usage estimated time is within the channel-usabletime. For example, in a case where the channel-usable time is relativelyshort, a faster wireless transmission rate is used when the wirelessbase station transmits a packet including data with low priority on thewireless network, whereby the first channel usage estimated time can bereduced, and data can be transmitted in real-time. On the other hand, ina case where the channel-usable time is relatively long, a slowerwireless transmission rate is used when the wireless base stationtransmits a packet including data with high priority on the wirelessnetwork, whereby the reliability of data transmission can be improved.The server also decides priority of data based on information relatingto the wireless terminal, so data suited to the wireless terminal can betransmitted with priority.

In the acquiring of information relating to the wireless terminal, afeedback packet including information relating to the wireless terminalmay be received from the wireless terminal.

According to this configuration, the server can easily acquireinformation relating to the wireless terminal by receiving a feedbackpacket from the wireless terminal.

A time interval for the wireless terminal to transmit the feedbackpacket to the server may be decided based on the number of the wirelessterminals connected to the wireless network.

According to this configuration, in a case where the number of wirelessterminals connected to the wireless network increases, increase in thechannel-used time can be suppressed by each of the wireless terminaltransmitting feedback packets, and channel-usable time can be secured.Accordingly, the server can transmit packets including more data to thewireless terminals via the wireless base station, using a wirelesstransmission rate with higher reliability.

A time interval for the wireless terminal to transmit the feedbackpacket to the server may be decided based on the channel-used time.

According to this configuration, the number of feedback packetstransmitted by the wireless terminal can be controlled in accordancewith increase/decrease in channel-used time. Thus, the server canmaintain the channel-used time constant, and can transmit a constantamount of packets to the wireless terminal via the wireless base stationusing a constant wireless transmission rate.

Information relating to the wireless terminal may include at least oneof radio wave reception information indicating a radio wave receptionstate of the wireless terminal, and demand information indicating dataof which the wireless terminal requests reception.

According to this configuration, the priority of data is decided basedon the state of the wireless terminal connected to the wireless network,so data suited to the wireless terminal can be transmitted withpriority, even in a case where the channel-usable time is short, forexample.

The communication method may further include setting an initial valuefor the wireless transmission rate. In the calculating of the wirelesstransmission rate, the initial value of the wireless transmission ratemay be changed based on the priority of the data included in aparticular packet, in a case where the first channel usage estimatedtime exceeds the channel-usable time.

According to this configuration, in a case where the first channel usageestimated time exceeds the channel-usable time, the initial value of thewireless transmission rate is changed based on the priority of data. Forexample, the first channel usage estimated time can be brought withinthe channel-usable time by changing the initial value of the wirelesstransmission rate corresponding to data with low priority to a highervalue, and data transmission can be performed in real-time.

In the generating, a plurality of the packets each including a pluralityof data of which the priority differs from each other may be generated.In the calculating of the first channel usage estimated time, the firstchannel usage estimated time may be calculated that is estimated time ofthe wireless base station using the channel to transmit the plurality ofpackets to the wireless terminal. In the calculating of the wirelesstransmission rate, in a case where the first channel usage estimatedtime exceeds the channel-usable time, the particular packet includingthe data of which the priority may be lowest is extracted from theplurality of packets, and the initial value for the wirelesstransmission rate corresponding to the particular packet may be changedto a higher value.

According to this configuration, in a case where the first channel usageestimated time exceeds the channel-usable time, the initial value of thewireless transmission rate corresponding to the particular packetincluding data with the lowest priority is changed to a higher value.Accordingly, the first channel usage estimated time can be broughtwithin the channel-usable time, and data can be transmitted inreal-time.

The communication method may further include: calculating, afterchanging the wireless transmission rate in the calculating of thewireless transmission rate, a second channel usage estimated time thatis estimated time of the wireless base station using the channel totransmit the plurality of packets including the particular packet to thewireless terminal; and determining whether or not to transmit theparticular packet to the wireless base station in a case where thesecond channel usage estimated time exceeds the channel-usable time.

According to this configuration, in a case where the second channelusage estimated time exceeds the channel-usable time, determination canbe made regarding whether or not to transmit the particular packet tothe wireless base station. For example, in a case where thechannel-usable time is extremely short, the server can transmit datawith high priority in real-time, by not transmitting the particularpacket including data with the lowest priority to the wireless basestation.

The communication method may further include discarding the particularpacket in a case of having determined not to transmit the particularpacket to the wireless base station in the determining.

According to this configuration, the particular packet can be suppressedfrom being accumulated at the wireless base station, by discarding theparticular packet.

In the transmitting, an identifier indicating the wireless transmissionrate may be imparted to the packet.

According to this configuration, information indicating the wirelesstransmission rate can be transmitted to the wireless base station by asimple method.

A server according to an aspect of the present disclosure is a serverthat transmits a packet by multicast or broadcast via a wireless basestation to at least one or more wireless terminals connected to thewireless base station via a wireless network using a channel. The serverincludes: a receiver that receives information from the wirelessterminal relating to the wireless terminal; a priority controller thatdecides priority of data based on information relating to the wirelessterminal; a generator that generates the packet including the dataregarding which the priority has been decided; an acquirer that acquireschannel-used time that is time over which the channel is used per unittime at the wireless base station; a first calculator that calculateschannel-usable time that is time over which the channel can be used perunit time, for the wireless base station to transmit the packet to thewireless terminal, based on the channel-used time; a second calculatorthat calculates a channel usage estimated time that is time estimated tobe used for the wireless base station to use the channel to transmit thepacket to the wireless terminal, and calculates a wireless transmissionrate to be applied when the wireless base station transmits the packetto the wireless terminal, based on the priority of the data included inthe packet, to where the channel usage estimated time is within thechannel-usable time; and a transmitter that transmits the packet, andinformation indicating the wireless transmission rate, to the wirelessbase station.

A wireless distribution system according to an aspect of the disclosureis a wireless distribution system including: a server; a wireless basestation connected to the server by a network; and at least one or morewireless terminals connected to the wireless base station via a wirelessnetwork using a channel. The server includes a first receiver thatreceives information from the wireless terminal relating to the wirelessterminal, a priority controller that decides priority of data based oninformation relating to the wireless terminal, a generator thatgenerates the packet including the data regarding which the priority hasbeen decided, an acquirer that acquires channel-used time that is timeover which the channel is used per unit time at the wireless basestation, a first calculator that calculates channel-usable time that istime over which the channel can be used per unit time, for the wirelessbase station to transmit the packet to the wireless terminal, based onthe channel-used time, a second calculator that calculates a channelusage estimated time that is time estimated to be used for the wirelessbase station to use the channel to transmit the packet to the wirelessterminal, and calculates a wireless transmission rate to be applied whenthe wireless base station transmits the packet to the wireless terminal,based on the priority of the data included in the packet, to where thechannel usage estimated time is within the channel-usable time, and afirst transmitter that transmits the packet, and information indicatingthe wireless transmission rate, to the wireless base station. Thewireless base station includes a second receiver that receives thepacket and the information from the server, a decider that decides thewireless transmission rate based on the information, and a secondtransmitter that transmits the packet by multicast or broadcast to thewireless terminal, at the decided wireless transmission rate.

It should be noted that general or specific embodiments may beimplemented as a system, a method, an integrated circuit, a computerprogram, a storage medium such as a computer-readable CD-ROM, or anyselective combination of the system, method, integrated circuit,computer program, and storage medium.

An embodiment will be described in detail below with reference to thedrawings. Note that the embodiments described below are all general orspecific examples of the present disclosure. Values, shapes, materials,components, arrangement and connection forms of components, steps,orders of steps, and so forth in the following embodiments are onlyexemplary, and do not restrict the present disclosure. Components in thefollowing embodiments which are not included in an independent Claimindicating the highest concept are described as being optionalcomponents.

Embodiment

1. Configuration of Wireless Distribution System

First, the configuration of a wireless distribution system according toan embodiment will be described with reference to FIG. 1. FIG. 1 is adiagram illustrating the configuration of the wireless distributionsystem according to the embodiment. The wireless distribution systemincludes a server 12, a wireless base station 13, and multiple wirelessterminals 14 a and 14 b, as illustrated in FIG. 1.

The server 12 is connected to the wireless base station 13 via a wirednetwork 10 (an example of a network). The server 12 receives feedbackpackets including information relating to each of the multiple wirelessterminals 14 a and 14 b, from each of the multiple wireless terminals 14a and 14 b via the wireless base station 13, and decide priority(described later) of data based in the received feedback packets. Theserver 12 transmits multicast packets (an example of a packet) includingdata regarding which the priority has been decided, to the multiplewireless terminals 14 a and 14 b via the wireless base station 13. Notethat the wired network 10 may be configured of one or more routers orone or more switches.

The wireless base station 13 configures a wireless network 11 using achannel. The wireless base station 13 receives a multicast packet fromthe server 12 and transmits the received multicast packet to themultiple wireless terminals 14 a and 14 b by multicast. The wirelessbase station 13 also receives feedback packets from each of the multiplewireless terminals 14 a and 14 b, and transmits the received feedbackpackets to the server 12.

The multiple wireless terminals 14 a and 14 b are each connected to thewireless base station 13 via the wireless network 11. The multiplewireless terminals 14 a and 14 b each receive multicast packets from thewireless base station 13. The multiple wireless terminals 14 a and 14 balso each transmit feedback packets, addressed to the server 12, to thewireless base station 13. The server 12 and wireless base station 13 maybe realized as a single physical device, with the wired network 10omitted.

2. Configuration of Wireless Base Station

Next, the configuration of the wireless base station 13 will bedescribed with reference to FIG. 2. FIG. 2 is a block diagramillustrating the configuration of the wireless base station 13 accordingto the embodiment.

The wireless base station 13 includes a wired communication unit 1301(an example of a second reception unit), a wireless transmission ratesetting unit 1302, a packet processing unit 1303 (an example of adeciding unit), a channel-used time measuring unit 1304 (an example of ameasuring unit), and a wireless communication unit 1305 (an example of atransmission unit and a second transmission unit), as illustrated inFIG. 2.

The wired communication unit 1301 is connected to the server 12 via thewired network 10. The wired communication unit 1301 receives multicastpackets from the server 12. The wired communication unit 1301 alsotransmits channel-used time (described later), measured by thechannel-used time measuring unit, 1304 to the server 12. The wiredcommunication unit 1301 also transmits feedback packets from each of themultiple wireless terminals 14 a and 14 b to the server 12.

The wireless transmission rate setting unit 1302 correlates identifiersincluded in multicast packets received by the wired communication unit1301 with the wireless transmission rate to be applied when transmittingmulticast packets on the wireless network 11. Note that this correlationis performed through the wired network 10 or wireless network 11. Anidentifier is at least one of, for example,

(a) Virtual LAN Identifier (VLAN ID) and Priority Code Point (PCP)stipulated by IEEE 802.1Q, that are included in an Ethernet frame,

(b) Type Of Service (TOS) included in an IPv4 header,

(c) Traffic Class included in an IPv6 header, and

(d) Source port No. and Destination port No. included in a User DatagramProtocol (UDP) header.

The wireless transmission rate setting unit 1302 performs correlation ofone of the wireless transmission rates of 6, 9, 13, 18, 24, 36, 48, and54 Mbps, to the identifier of the multicast packet based on the IEEE802.11a standard, for example. Note that the wireless transmission ratesetting unit 1302 may correlate a wireless transmission rate set to afixed value beforehand, to the identifier of the multicast packet.

The packet processing unit 1303 decides the wireless transmission rateto be correlated by the wireless transmission rate setting unit 1302when transmitting a multicast packet on the wireless network 11, basedon the identifier included in the multicast packet. The packetprocessing unit 1303 also outputs feedback packets received by thewireless communication unit 1305 to the wired communication unit 1301.

The channel-used time measuring unit 1304 measures channel-used time,which is time where a channel is used per unit time (e.g., 100 msec)other than for transmission of multicast packets.

The wireless communication unit 1305 is connected to the multiplewireless terminals 14 a and 14 b via the wireless network 11. Thewireless communication unit 1305 uses the wireless transmission ratedecided by the packet processing unit 1303 to transmit a multicastpacket to the multiple wireless terminals 14 a and 14 b by multicast.The wireless communication unit 1305 also receives feedback packets fromeach of the multiple wireless terminals 14 a and 14 b.

3. Server Configuration

Next, the configuration of the server 12 will be described withreference to FIGS. 3 through 5. FIG. 3 is a block diagram illustratingthe configuration of the server 12 according to the embodiment. FIG. 4is a diagram illustrating an example of a wireless transmission ratecontrol management table 30. FIG. 5 is a diagram illustrating an exampleof a wireless terminal management table 50.

The server 12 includes a data generating unit 1201, a packet generatingunit 1202 (an example of a generating unit), a wireless transmissionrate calculating unit 1203 (an example of a second calculating unit), apacket transmission unit 1204, a channel-usable time calculating unit1205 (an example of a first calculating unit), a channel-used timeacquisition unit 1206 (an example of an acquisition unit), a wiredcommunication unit 1207 (an example of a transmission unit and a firsttransmission unit), a feedback reception unit 1208 (an example of areception unit and a first reception unit), a wireless terminalmanagement table storage unit 1209, and a priority control unit 1210, asillustrated in FIG. 3.

The data generating unit 1201 generates data of which the priority hasbeen decided by the priority control unit 1210. Note that priority isinformation indicating the order of priority in which data istransmitted, and is made up of three stages of “high”, “medium”, and“low”, for example. In a case where the priority set to the data is“high”, for example, the order or priority of transmission of this datais highest. In a case where the priority set to the data is “medium”,the order or priority of transmission of this data is second highest. Ina case where the priority set to the data is “low”, the order orpriority of transmission of this data is lowest.

The packet generating unit 1202 generates multicast packets includingthe data generated by the data generating unit 1201. Note that themulticast IP addresses specified by the server 12 as the destination forthe multicast packets may all be the same. Accordingly, in a case wherethe wired network 10 is configured of one or more routers or one or moreswitches, the number of times and the amount of time regardingconfiguration of a distribution tree to transmit the multicast packetson the wired network 10 can be reduced as compared to a case of usingmultiple multicast IP addresses. As a result, the multiple wirelessterminals 14 a and 14 b can promptly receive the multicast packetstransmitted by the server 12.

The packet generating unit 1202 may provide the multicast packets with atime interval for each of the multiple wireless terminals 14 a and 14 bto transmit feedback packets addressed to the server 12. The timeinterval for transmitting feedback packets in this case may be decidedbased on the number of wireless terminals connected to the wirelessnetwork 11, stored in the wireless terminal management table 50(described later). Alternatively, the time interval for transmittingfeedback packets may be decided based on the channel-used time acquiredfrom the channel-used time acquisition unit 1206.

The channel-used time acquisition unit 1206 acquires channel-used time,measured by the channel-used time measuring unit 1304 of the wirelessbase station 13, from the wireless base station 13.

The channel-usable time calculating unit 1205 calculates thechannel-usable time, which is time that the channel can be used per unitfor the wireless base station 13 to transmit multicast packets, based onthe channel-used time acquired by the channel-used time acquisition unit1206. Specifically, the channel-usable time calculating unit 1205calculates the channel-usable time using, for example, the followingExpression (1),T _(usable)=α×(T−T _(used))  Expression (1)where T_(usable) represents the channel-usable time, α represents a realnumber that is greater than 0 but no greater than 1, T represents unittime for the wireless base station 13 to measure the channel-used time,and T_(used) represents the channel-used time. Note that thechannel-used time may be the average value of the channel-used timemeasured this time and channel-used time measured in the past.

The wireless transmission rate calculating unit 1203 calculates channelusage estimated time. The channel usage estimated time is estimated timeof using the channel for the wireless base station 13 to transmit amulticast packet to the multiple wireless terminals 14 a and 14 b usinga wireless transmission rate. Specifically, the wireless transmissionrate calculating unit 1203 calculates the channel usage estimated timebased on a standard stipulated in IEEE 802.11a for example, using thefollowing Expressions (2) and (3)

$\begin{matrix}{T_{est} = {{N \times \left( {t_{DIFS} + t_{backoff} + t_{{PLCP}\;\_\;{preamble}} + t_{{PLCP}\;\_\;{signal}}} \right)} + {\sum\limits_{n = 1}^{N}t_{variable}}}} & {{Expression}\mspace{11mu}(2)} \\{t_{variable} = {\frac{S_{{PLCP}\;\_\;{service}} + S_{PSDU} + S_{tail} + S_{padding}}{S_{symble}} \times t_{symble}}} & {{Expression}\mspace{14mu}(3)}\end{matrix}$where, in Expression (2), T_(est) represents channel usage estimatedtime, N represents the number of multicast packets including data towhich priority has been set, t_(DIFS) represents a time interval for DCFInter Frame Space (IFS), t_(backoff) represents an average backoff timeinterval based on Contention Window, t_(PLCP) _(_) _(preamble)represents time required for preamble transmission of Physical LayerConvergence Protocol (PLCP) on the wireless network 11, t_(PLCT) _(_)_(signal) represents time required for transmission of the signalportion of the PLCP header on the wireless network 11, and t_(variable)represents time required in a case of transmitting bits from the serviceportion up to the padding of a later-described PLCP header at a wirelesstransmission rate on the wireless network 11.

Also in Expression (3), S_(PLCP) _(_) _(service) represents the bitcount of the service portion of the PLCP header, S_(PSDU) represents thebit count of a media access control (MAC) protocol data unit (PDU)including multicast packets, S_(tail) represents the bit count of tailbits indicating the end of the frame, S_(padding) represents the bitcount of the padding portion, S_(symble) represents the bit count persymbol in a case of transmitting the frame using the wirelesstransmission rate on the wireless network 11, and t_(symble) representsthe amount of time require for transmission of one symbol on thewireless network 11.

The wireless transmission rate calculating unit 1203 calculates awireless transmission rate to be applied for when transmitting multicastpackets, based on the number and size of the multicast packets, andpriority of data included in the multicast packet, so that the channelusage estimated time is within the channel-usable time. In a case wherethe wireless transmission rate to be applied for when transmittingmulticast packets cannot be calculated so that the channel usageestimated time is within the channel-usable time, the wirelesstransmission rate calculating unit 1203 determines whether or not totransmit the multicast packets to the wireless base station 13, based onthe priority of data included in the multicast packet.

Further, the wireless transmission rate calculating unit 1203 stores awireless transmission rate control management table 30 that storesinformation relating to multicast packets to be transmitted. Thewireless transmission rate control management table 30 will be describednow with reference to FIG. 4. It can be seen from FIG. 4 that thewireless transmission rate control management table 30 includes apriority column 301, a control object column 302, a transmission objectcolumn 303, wireless transmission rate column 304, as illustrated inFIG. 4.

The priority column 301 stores the priority of data included in themulticast packet. In the example illustrated in FIG. 4, information of“high”, indicating the highest priority, is stored in the first row ofthe priority column 301. Information of “medium”, indicating secondhighest priority, is stored in the second row of the priority column301, information of “low”, indicating the lowest priority, is stored inthe third row of the priority column 301.

The control object column 302 stores information indicating whether ornot the object of changing the wireless transmission rate to be appliedwhen transmitting the multicast packet. In the example illustrated inFIG. 4, the first row of the control object column 302 storesinformation of “false”, indicating that this is not an object ofchanging the wireless transmission rate to be applied when transmittingthe multicast packet. The second and third rows of the control objectcolumn 302 store information of “true”, indicating that this is anobject of changing the wireless transmission rate to be applied whentransmitting the multicast packet.

The transmission object column 303 stores information indicating whetheror not the object of transmitting the multicast packet. In the examplein FIG. 4, the first through third rows of the transmission objectcolumn 303 store information of “true”, indicating that this is anobject of transmission of the multicast packet.

The wireless transmission rate column 304 stores information indicatingthe wireless transmission rate to be applied when transmitting themulticast packet. In the example illustrated in FIG. 4, the first row ofthe wireless transmission rate column 304 stores information of“OFDM6Mbps”, indicating that the wireless transmission rate is 6 Mbps.The second row of the wireless transmission rate column 304 storesinformation of “OFDM12Mbps”, indicating that the wireless transmissionrate is 12 Mbps. The third row of the wireless transmission rate column304 stores information of “OFDM24Mbps”, indicating that the wirelesstransmission rate is 24 Mbps.

In a case where a multicast packet including data to which apredetermined priority has been set is to be transmitted at a wirelesstransmission rate initial value set beforehand without fail at thewireless base station 13, this can be realized by setting the controlobject column 302 to “false” and the wireless transmission rate column304 to the initial value of the wireless transmission rate.

Returning to FIG. 3, the packet transmission unit 1204 imparts themulticast packet to be transmitted with an identifier corresponding tothe wireless transmission rate (an example of information indicatingwireless transmission rate), based on the calculation results of thewireless transmission rate calculating unit 1203.

The wired communication unit 1207 is connected to the wireless basestation 13 via the wired network 10. The wired communication unit 1207transmits the multicast packet processed by the packet transmission unit1204 to the wireless base station 13. The identifier corresponding tothe wireless transmission rate has been imparted to the multicast packetat this time, so by transmitting the multicast packet to the wirelessbase station 13, the wired communication unit 1207 is notifying thewireless base station 13 of the information indicating the wirelesstransmission rate. The wired communication unit 1207 also receiveschannel-used time from the wireless base station 13, and outputs thereceived channel-used time to the channel-used time acquisition unit1206. The wired communication unit 1207 also receives feedback packetsfrom the wireless base station 13 and outputs the receives feedbackpackets to the feedback reception unit 1208. The feedback reception unit1208 receives the feedback packets from the wireless base station 13 viathe wired communication unit 1207.

The wireless terminal management table storage unit 1209 stores thewireless terminal management table 50 storing information relating toeach of the multiple wireless terminals 14 a and 14 b included in thefeedback packets. In a case of the server 12 transmitting three types ofvideo data, with different resolution, for example, the wirelessterminal management table 50 is used to decide the priority of databased on demand information (described later) relating to video displaycapabilities of each of the multiple wireless terminals 14 a and 14 b.

Now, the wireless terminal management table 50 will be described withreference to FIG. 5. The wireless terminal management table 50 includesa video data type column 501, a resolution column 502, and a wirelessterminal list column 503, as illustrated in FIG. 5.

The video data type column 501 stores information relating to the typeof video data. In the example illustrated in FIG. 5, the first row ofthe video data type column 501 stores information of “video data A”. Thesecond row of the video data type column 501 stores information of“video data B”. The third row of the video data type column 501 storesinformation of “video data C”.

The resolution column 502 stores information relating to resolution ofvideo data. In the example illustrated in FIG. 5, the first row of theresolution column 502 stores information of “1920×1080”, which is theresolution of video data A. The second row of the resolution column 502stores information of “1280×720”, which is the resolution of video dataB. The third row of the resolution column 502 stores information of“720×480”, which is the resolution of video data C.

The wireless terminal list column 503 stores information relating to thewireless terminals requesting reception of video data stored in thevideo data type column 501. In the example illustrated in FIG. 5, thefirst row of the wireless terminal list column 503 stores informationindicating that there is no wireless terminal that requests reception ofthe video data A. The second row of the wireless terminal list column503 stores information of “wireless terminal 14 a” indicating thatwireless terminal 14 a requests reception of the video data B. The thirdrow of the wireless terminal list column 503 stores information of“wireless terminal 14 b” indicating that wireless terminal 14 b requestsreception of the video data C.

The priority control unit 1210 decides priority of data based oninformation relating to each of the multiple wireless terminals 14 a and14 b. Specifically, the priority control unit 1210 decides the priorityof data using the wireless terminal management table 50.

4. Configuration of Wireless Terminal

Next, the configuration of the wireless terminals 14 a and 14 b will bedescribed with reference to FIG. 6. FIG. 6 is a block diagramillustrating the configuration of the wireless terminals 14 a and 14 baccording to the present embodiment. Note that the configurations of thewireless terminals 14 a and 14 b are the same, so just the configurationof the wireless terminal 14 a will be described here.

The wireless terminal 14 a includes a wireless communication unit 1401,a packet reception unit 1402, a data processing unit 1403, a feedbackgenerating unit 1404, and a feedback transmission unit 1405, asillustrated in FIG. 6.

The wireless communication unit 1401 is connected to the wireless basestation 13 via the wireless network 11. The wireless communication unit1401 receives multicast packets from the wireless base station 13. Thewireless communication unit 1401 transmits feedback packets from thefeedback transmission unit 1405 that are addressed to the server 12, tothe wireless base station 13.

The packet reception unit 1402 uses an IGMP join message to receive amulticast packet addressed to a multicast IP address transmitted by theserver 12, from the wireless communication unit 1401. The packetreception unit 1402 outputs the data included in the received multicastpacket to the data processing unit 1403. The data processing unit 1403processes the data included in the multicast packet from the packetreception unit 1402, and outputs to the feedback generating unit 1404.

The feedback generating unit 1404 generates information relating to thewireless terminal 14 a at predetermined time intervals, and generates afeedback packet containing this information. The information relating tothe wireless terminal 14 a includes demand information indicating datathat that the wireless terminal 14 a requests to receive, relating to atleast two types or more of data that the server 12 transmits, forexample. Specifically, demand information is information that indicatesvideo data of resolution that is suitable of video display capabilitiesof each of the multiple wireless terminals 14 a and 14 b, in a casewhere the server 12 transmits video data of two or more types ofresolution, for example.

Note that the information relating to the wireless terminal 14 a mayinclude radio wave reception information indicating the radio wavereception state of the wireless terminal 14 a. Representative examplesof radio wave reception information include received signal strengthindicator (RSSI) and signal-to-noise ratio (SNR) or the like.

The feedback generating unit 1404 acquires the radio wave receptioninformation from the wireless communication unit 1401. The feedbackgenerating unit 1404 acquires demand information from the dataprocessing unit 1403. Note that demand information may be set by theuser using the wireless terminal 14 a. Alternatively, the demandinformation may be set based on the display capabilities of the displayof the first portion 14 such as resolution or the like, capabilities ofthe central processing unit (CPU), or data processing capabilities suchas the current processing load of the CPU or the like.

The feedback transmission unit 1405 transmits the feedback packetsgenerated by the feedback generating unit 1404 to the wireless basestation 13 via the wireless communication unit 1401.

Note that the time interval for generating and transmitting feedbackpackets may be decided by the server 12 based on the channel-used time,for example. Alternatively, the time interval for generating andtransmitting feedback packets may be decided by the wireless terminal 14a, based on the number of the multiple wireless terminals 14 a and 14 bconnected to the wireless network 11, for example.

5. Processing by Server

Processing that the server 12 performs in the embodiment will bedescribed with reference to FIGS. 7 through 10C. FIGS. 7 and 8 areflowchart illustrating processing performed by the server 12 accordingto the present embodiment. FIG. 9 is a flowchart illustrating thespecifics of processing performed in wireless transmission rate changingprocessing (S20) in FIG. 8. FIGS. 10A through 10C are diagrams fordescribing an example of the wireless transmission rate changingprocessing in FIG. 8. Note that FIGS. 10A through 10C assume that α=1 inthe above Expression (1), and that the number and size is the same foreach of multicast packets P1, P2, and P3, to simplify description.

First, the priority control unit 1210 decides the priority of data,using the wireless terminal management table 50. An example of datapriority deciding processing by the priority control unit 1210 will bedescribed with reference to FIG. 7.

As illustrated in FIG. 7, the priority control unit 1210 counts thenumber of wireless terminals stored in the wireless terminal list column503, for each type of video data stored in the video data type column501 in the wireless terminal management table 50 (S1)

Next, the priority control unit 1210 imparts each type of video datastored in the video data type column 501 priority in the order of“high”, “medium”, and “low”, from that which has more wireless terminalscounted in S1, thereby deciding priority (S2).

In the example illustrated in FIG. 5, there is one wireless terminalstored in the wireless terminal list column 503 for each of video data Band C. In a case where the demand information is information indicatingvideo data of a resolution suitable for the video display capabilitiesof the displays of each of the wireless terminals 14 a and 14 b, forexample, the priority control unit 1210 decides that the priorityrelating to video data C of the resolution that both of the wirelessterminals 14 a and 14 b can display is “high”, the priority relating tovideo data B of the resolution that just the wireless terminal 14 a candisplay is “medium”, and the priority relating to video data A of theresolution that neither of the wireless terminals 14 a and 14 b candisplay is “low”.

Next, an example of multicast packet transmission processing will bedescribed with reference to FIG. 8. The packet generating unit 1202first generates multicast packets containing data generated by the datagenerating unit 1201 (S11), as illustrated in FIG. 8. Specifically, thepriority of data is made up of the three levels of “high”, “medium”, and“low”, and the packet generating unit 1202 generates multicast packetsP1 containing data of which the priority is “high”, multicast packets P2containing data of which the priority is “medium”, and multicast packetsP3 containing data of which the priority is “low”, as illustrated inFIGS. 10A through 10C. In the example illustrated in FIG. 5, the data ofwhich the priority is “high” is the video data C, the data of which thepriority is “medium” is the video data B, and the data of which thepriority is “low” is the video data A.

The wireless transmission rate calculating unit 1203 counts the numberand size of multicast packets for each of the priorities “high”,“medium”, and “low”, of data contained in the multicast packets P1, P2,and P3 generated by the packet generating unit 1202 (S12).

Next, the wireless transmission rate calculating unit 1203 sets initialvalues R1, R2, and R3, of wireless transmission rate, for each of thepriorities “high”, “medium”, and “low”, of data contained in themulticast packets P1, P2, and P3, of which the transmission objectcolumn 303 illustrated in FIG. 4 indicates “true” (S13). That is to say,the wireless transmission rate calculating unit 1203 sets the initialvalue R1 for the wireless transmission rate for the priority “high”, theinitial value R2 for the wireless transmission rate for the priority“medium”, and the initial value R3 for the wireless transmission ratefor the priority “low”. The initial values R1, R2, and R3 for thewireless transmission rate will all be assumed to be the same valueshere, for the sake of description.

Next, the channel-used time acquisition unit 1206 acquires thechannel-used time T_(used) from the wireless base station 13 (S14). Thechannel-usable time calculating unit 1205 calculates the channel-usabletime T_(usable) based on the channel-used time T_(used) acquired by thechannel-used time acquisition unit 1206 (S15).

Next, the wireless transmission rate calculating unit 1203 calculatesthe channel usage estimated time based on the initial values R1, R2, andR3 for the wireless transmission rate, and number and size of themulticast packets P1, P2, and P3 (an example of first channel usageestimated time) (S16).

Next, the wireless transmission rate calculating unit 1203 determineswhether or not the channel usage estimated time is within thechannel-usable time T_(usable) (S17). In a case where the wirelesstransmission rate calculating unit 1203 determines that the channelusage estimated time T_(est) _(_) ₁ is within the channel-usable timeT_(usable) (YES in S17) such as illustrated in FIG. 10A, the initialvalues R1, R2, and R3 for the wireless transmission rate are maintainedwithout being changed. The packet transmission unit 1204 impartsidentifiers corresponding to the initial values R1, R2, and R3 for thewireless transmission rate, to the multicast packets P1, P2, and P3.Accordingly, the wired communication unit 1207 transmits each of themulticast packets P1, P2, and P3 processed by the packet transmissionunit 1204 to the wireless base station 13 at the respective initialvalues R1, R2, and R3 for the wireless transmission rate (S18).

On the other hand, in a case where the wireless transmission ratecalculating unit 1203 determines that the channel usage estimated timeT_(est) _(_) ₂ exceeds the channel-usable time T_(usable) (NO in S17)such as indicated by (a) in FIG. 10B and (a) in FIG. 10C, the wirelesstransmission rate calculating unit 1203 searches the wirelesstransmission rate control management table 30 and determines whether ornot there exists a priority for data contained in the multicast packetsP1, P2, and P3, where the control object column 302 and transmissionobject column 303 both are “true” (S19). That is to say, the wirelesstransmission rate calculating unit 1203 determines whether or not thereexist multicast packets that are the object of control where thewireless transmission rate is changed, and also the object oftransmission.

In the examples illustrated in FIGS. 4, 10B, and 10C, the wirelesstransmission rate calculating unit 1203 determines that there existmulticast packets P2 and P3 that are the object of control and also theobject of transmission (YES in S19). In this case, wireless transmissionrate changing processing based on the priority is executed (S20).Wireless transmission rate changing processing will be described laterin detail.

On the other hand, in a case where the wireless transmission ratecalculating unit 1203 determines that there are no multicast packetsthat are the object of control and also the object of transmission,i.e., that at least one of the control object column 302 andtransmission object column 303 give “false” for the priority of datacontained in all of the multicast packets P1, P2, and P3 in the wirelesstransmission rate control management table 30 (NO in S19), theprocessing ends.

Now, the above-described wireless transmission rate changing processing(S20) will be described in detail with reference to FIG. 9. The wirelesstransmission rate calculating unit 1203 searches the wirelesstransmission rate control management table 30 and extracts a multicastpacket containing data of which the priority is lowest and where bothcontrol object column 302 and transmission object column 303 give “true”(an example of a particular packet) (S41), as illustrated in FIG. 9. Inthe example illustrated in FIGS. 4. 10B, and 10C, the wirelesstransmission rate calculating unit 1203 extracts the multicast packet P3containing data of which the priority is “low”.

Next, the wireless transmission rate calculating unit 1203 determineswhether or not the initial value R3 of the wireless transmission ratecorresponding to the priority “low” of the extracted multicast packet P3in the wireless transmission rate control management table 30 can bechanged to a higher value (S42). In a case of having determined that theinitial value R3 of the wireless transmission rate can be changed to ahigher value (YES in S42), the wireless transmission rate calculatingunit 1203 changes the initial value R3 of the wireless transmission rateto a value R3′ (R3<R3′) that is higher (S43).

Next, the wireless transmission rate calculating unit 1203 calculatesthe channel usage estimated time based on the initial values R1, R2, andchanged value R3′ for the wireless transmission rate, and number andsize of the multicast packets P1, P2, and P3 (an example of secondchannel usage estimated time) (S44). Note that the calculation methodfor the channel usage estimated time in step S44 is the same as thecalculation method for the channel usage estimated time in step S16described above.

Next, the wireless transmission rate calculating unit 1203 determineswhether or not the channel usage estimated time is within thechannel-usable time T_(usable) (S45). In a case where the wirelesstransmission rate calculating unit 1203 determines that the channelusage estimated time T_(est) _(_) ₃ is within the channel-usable timeT_(usable) (YES in S45) as illustrated in FIG. 10B, the flow advances tostep S21 in FIG. 8, and determination is made in step S21 regardingwhether or not the multicast packet P3 containing data of which thepriority is lowest has been discarded.

Returning to FIG. 8, in a case where the wireless transmission ratecalculating unit 1203 has determined that the multicast packet P3containing data of which the priority is lowest has not been discarded(NO in S21), the packet transmission unit 1204 is notified of theinitial values R1 and R2 and the changed value R3′ of the wirelesstransmission rate. The packet transmission unit 1204 imparts identifierscorresponding to the initial values R1 and R2 for the wirelesstransmission rate to the multicast packets P1 and P2, and imparts anidentifier corresponding to the changed value R3′ for the wirelesstransmission rate to the multicast packet P3. Thus, the wiredcommunication unit 1207 transmits the multicast packets P1 and P2 to thewireless base station 13 at the respective initial values R1 and R2 forthe wireless transmission rate, and transmits the multicast packet P3 tothe wireless base station 13 at the changed value R3′ for the wirelesstransmission rate.

Returning to step S45 in FIG. 9, in a case where the wirelesstransmission rate calculating unit 1203 determines that the channelusage estimated time T_(est) _(_) ₄ exceeds the channel-usable timeT_(usable) (NO in S45) as indicated by (b) in FIG. 10C, the flow returnsto step S42. Accordingly, the wireless transmission rate calculatingunit 1203 determines whether or not the changed value R3′ of thewireless transmission rate corresponding to the priority “low” of themulticast packet P3 can be changed to a higher value (S42). The wirelesstransmission rate calculating unit 1203 repeatedly executes each processin the above-described steps S42 through S45 until the channel usageestimated time is within the channel-usable time T_(usable).

On the other hand, in a case where determination is made that theinitial value R3 or changed value R3′ of the wireless transmission ratecorresponding to the priority “low” of the multicast packet P3 cannot bechanged to a higher value (NO in S42), the wireless transmission ratecalculating unit 1203 excludes the multicast packet P3 containing thedata of which the priority is the lowest from the objects oftransmission (S46), as indicated by (c) in FIG. 10C. Specifically, thewireless transmission rate calculating unit 1203 changes thetransmission object column 303 corresponding to the priority “low” inthe wireless transmission rate control management table 30 from “true”to “false”. A conceivable example of a case where the initial value R3or changed value R3′ of the wireless transmission rate corresponding tothe priority “low” of the multicast packet P3 cannot be changed to ahigher value is a case where the initial value R3 or changed value R3′of the wireless transmission rate is 54 Mbps, which is the upper limitvalue in the IEEE 802.11a standard. The wireless transmission ratecalculating unit 1203 discards the multicast packet P3 containing thedata of which the priority is the lowest, which has been excluded fromthe object of transmission (S47), and the flow advances to step S21 inFIG. 8.

Returning to FIG. 8, in a case where the wireless transmission ratecalculating unit 1203 determines that the multicast packet P3 containingthe data of which the priority is the lowest has been discarded in stepS20 (YES in S21), the flow returns to step S16, where the channel usageestimated time T_(est) _(_) ₅ is calculated based on the initial valuesR1 and R2 for the wireless transmission rate, and number and size of themulticast packets P1 and P2 (S16). In a case where the channel usageestimated time T_(est) _(_) ₅ is within the channel-usable timeT_(usable) as indicated by (c) in FIG. 10C (YES in S17), the packettransmission unit 1204 imparts identifiers corresponding to the initialvalues R1 and R2 for the wireless transmission rate as to the multicastpackets P1 and P2. Accordingly, the wired communication unit 1207transmits the multicast packets P1 and P2 to the wireless base station13 at the respective initial values R1 and R2 for the wirelesstransmission rate (S18).

Note that in a case where the channel usage estimated time T_(est) _(_)₅ exceeds the channel-usable time T_(usable) (NO in FIG. 17), theprocesses in the above-described steps S16, S17, S20, and S21 arerepeatedly executed until the channel usage estimated time is within thechannel-usable time T_(usable).

6. Advantages

As described above, the server 12 controls the wireless transmissionrate based on the priority of data included in the multicast packets, sothat the channel usage estimated time is within the channel-usable time.For example, in a case where the channel-usable time is relativelyshort, a faster wireless transmission rate is used when the wirelessbase station 13 transmits, on the wireless network 11, packets includingdata of which the priority is low, thereby reducing the channel usageestimated time, and as a result, data can be transmitted in real-time.The server 12 decides the priority of data based on information relatingto each of the wireless terminals 14 a and 14 b, and accordingly cantransmit data suitable for each of the wireless terminals 14 a and 14 bwith priority.

Other Embodiments

While a communication method and so forth according to one or multipleaspects have been described by way of the above embodiment, the presentdisclosure is not restricted to the above embodiment. Variousmodifications to the embodiment and combinations of components ofdifferent embodiments which are conceivable by one skilled in the artmay be encompassed by one or multiple aspects without departing from theessence of the present disclosure.

Although description has been made in the embodiment above regarding acase where the server 12 transmits multicast packets to multiplewireless terminals 14 a and 14 b by multicast, this is not restrictive.For example, broadcast packets (an example of a packet) may betransmitted to multiple wireless terminals 14 a and 14 b by broadcast.

Although description has been made in the embodiment above regarding acase where the wireless distribution system has two wireless terminals14 a and 14 b, this is not restrictive. For example, the number ofwireless terminals that the wireless distribution system has may be one,or three or more.

Although description has been made in the embodiment above regarding aconfiguration where priority is in the three levels of “high”, “medium”,and “low”, this not restrictive. For example, priority may be in twolevels, or four or more levels.

Note that the components of the above embodiments may be realized bydedicated hardware, or may be realized by a software program suitablefor the components being executed. The components may be realized by aprogram executing unit such as a CPU or another processor or the likereading out and executing the software program recorded in a recordingmedium such as a hard disk, semiconductor memory, or the like. Softwarerealizing the server and so forth in the above embodiments is a programsuch as follows.

That is to say, the program causes a computer to execute acquiring ofinformation relating to the wireless terminal; deciding of priority ofdata based on information relating to the wireless terminal; generatingthe packet including the data regarding which the priority has beendecided; acquiring of channel-used time that is time over which thechannel is used per unit time at the wireless base station; calculatingof channel-usable time that is time over which the channel can be usedper unit time, for the wireless base station to transmit the packet tothe wireless terminal, based on the channel-used time; calculating of afirst channel usage estimated time that is time estimated to be used forthe wireless base station to use the network to transmit the packet tothe wireless terminal; calculating of a wireless transmission rate to beapplied when the wireless base station transmits the packet to thewireless terminal, based on the priority of the data included in thepacket, to where the first channel usage estimated time is within thechannel-usable time; and transmitting of the packet, and informationindicating the wireless transmission rate, to the wireless base station.

Part or all of the components of which the above-described devices areconfigured may be configured as an IC card detachably mountable to eachdevice, or a standalone module. The IC card or the module is a computersystem configured including a microprocessor, ROM, RAM, and so forth.The IC card or the module may include a super-multifunctional LSI. TheIC card or the module achieves its functions by the microprocessoroperating according to the computer program. The IC card or the modulemay be tamper-resistant.

The present disclosure may in one form be the above-described method,may be a computer program which realizes these methods by a computer, ormay be digital signals made up of the computer program. The presentdisclosure may in one form be the computer program or the digitalsignals recorded in a computer-readable recording medium, such as forexample, a flexible disk, hard disk, CD-ROM, MO, DVD, DVD-ROM, DVD-RAM,BD (Blu-ray (a registered trademark) Disc), semiconductor memory, or thelike. The present disclosure may also be the digital signals recorded inthese recording mediums. The present disclosure may in one form be anarrangement where the computer program or the digital signals aretransmitted over an electric communication line, wireless or wiredcommunication line, a network of which the Internet is representative,data broadcasting, or the like. The present disclosure may in one formbe a computer system having a microprocessor and memory, where thememory stores the computer program, and the microprocessor operatesaccording to the computer program. This may also be carried out byanother independent computer system, by the program or digital signalsbeing recorded in the recording medium and being transported, or by theprogram or digital signals being transported over the network or thelike.

The communication method, server, and wireless distribution systemaccording to the present disclosure are useful in a system or the likethat distributes in real-time dynamic maps to a self-driving vehicle forexample, indicating traffic states and the like.

What is claimed is:
 1. A communication method of a server that transmitsa packet by multicast or broadcast via a wireless base station to atleast one or more wireless terminals connected to the wireless basestation via a wireless network using a channel, the method comprising:acquiring, by a processor, a state of a first wireless terminal;deciding, by the processor, a priority to be assigned to a packetdirected to the first wireless terminal based on the acquired state ofthe first wireless terminal; generating, by the processor, the packetdirected to the first wireless terminal, the packet including prioritydata regarding the priority assigned to the packet directed to the firstwireless terminal; acquiring, by the processor, channel-used time thatis time over which the channel is used per unit time at the wirelessbase station; calculating, by the processor, channel-usable time that istime over which the channel can be used per unit time, for the wirelessbase station to transmit the packet to the first wireless terminal,based on the channel-used time; calculating, by the processor, a firstchannel usage estimated time that is time estimated to be used for thewireless base station to use the channel to transmit the packet to thefirst wireless terminal; calculating, by the processor, a wirelesstransmission rate to be applied when the first wireless base stationtransmits the packet to the first wireless terminal, based on thepriority of the data included in the packet, to where the first channelusage estimated time is within the channel-usable time; andtransmitting, by a transmitter, the packet, and information indicatingthe wireless transmission rate, to the wireless base station, whereinthe packet is transmitted, via the wireless base station, to the firstwireless terminal in accordance with the calculated wirelesstransmission rate, and wherein the wireless transmission rate calculatedfor the packet directed to the first wireless terminal is different froma wireless transmission rate calculated for a packet directed to asecond wireless terminal that is in a different state than the firstwireless terminal.
 2. The communication method according to claim 1,wherein, in the acquiring of the state of the first wireless terminal, afeedback packet including information relating to the first wirelessterminal is received from the first wireless terminal.
 3. Thecommunication method according to claim 2, wherein a time interval forthe first wireless terminal to transmit the feedback packet to theserver is decided based on a number of the wireless terminals connectedto the wireless network.
 4. The communication method according to claim2, wherein a time interval for first the wireless terminal to transmitthe feedback packet to the server is decided based on the channel-usedtime.
 5. The communication method according to claim 1, wherein thestate of the first wireless terminal includes at least one of radio wavereception information indicating a radio wave reception state of thefirst wireless terminal, and demand information indicating priority dataof which the first wireless terminal requests reception.
 6. Thecommunication method according to claim 1, further comprising: setting,by the processor, an initial value for the wireless transmission ratefor the packet directed to the first wireless terminal, wherein, in thecalculating of the wireless transmission rate for the packet directed tothe first wireless terminal, the initial value of the wirelesstransmission rate is changed based on the priority indicated in thepriority data included in a particular packet, in a case where the firstchannel usage estimated time exceeds the channel-usable time.
 7. Thecommunication method according to claim 6, wherein, in the generating, aplurality of the packets each including a plurality of priority data ofwhich priority differs from each other are generated, wherein, in thecalculating of the first channel usage estimated time, the first channelusage estimated time is calculated that is estimated time of thewireless base station using the channel to transmit the plurality ofpackets to the first wireless terminal, and wherein, in the calculatingof the wireless transmission rate for the packet directed to the firstwireless terminal, in a case where the first channel usage estimatedtime exceeds the channel-usable time, the particular packet includingthe data of which the priority is lowest is extracted from the pluralityof packets, and the initial value for the wireless transmission ratecorresponding to the particular packet is changed to a higher value. 8.The communication method according to claim 6, further comprising:calculating, by the processor and after changing the wirelesstransmission rate in the calculating of the wireless transmission rate,a second channel usage estimated time that is estimated time of thewireless base station using the channel to transmit the plurality ofpackets including the particular packet to the first wireless terminal;and determining, by the processor, whether or not to transmit theparticular packet to the wireless base station in a case where thesecond channel usage estimated time exceeds the channel-usable time. 9.The communication method according to claim 8, further comprising:discarding, by the processor, the particular packet in a case of havingdetermined not to transmit the particular packet to the wireless basestation in the determining.
 10. The communication method according toclaim 1, wherein, in the transmitting, an identifier indicating thewireless transmission rate for the packet directed to the first wirelessterminal is imparted to the packet.
 11. A server that transmits a packetby multicast or broadcast via a wireless base station to at least one ormore wireless terminals connected to the wireless base station via awireless network using a channel, the server comprising: a receiver thatreceives, from a first wireless terminal, a state of the first wirelessterminal; a priority controller that decides a priority to be assignedto a packet directed to the first wireless terminal based on thereceived state of the first wireless terminal; a generator thatgenerates the packet directed to the first wireless terminal includingpriority data indicating the assigned priority; an acquirer thatacquires channel-used time that is time over which the channel is usedper unit time at the wireless base station; a first calculator thatcalculates channel-usable time that is time over which the channel canbe used per unit time, for the wireless base station to transmit thepacket to the first wireless terminal, based on the channel-used time; asecond calculator that calculates a channel usage estimated time that istime estimated to be used for the wireless base station to use thechannel to transmit the packet to the first wireless terminal, andcalculates a wireless transmission rate to be applied when the wirelessbase station transmits the packet to the first wireless terminal, basedon the priority indicated by the priority data included in the packet,to where the channel usage estimated time is within the channel-usabletime; a transmitter that transmits the packet, and informationindicating the wireless transmission rate, to the wireless base station,wherein the information indicating the wireless transmission rate causesthe wireless base station to transmit the packet to the first wirelessterminal in accordance with the calculated wireless transmission rate,and wherein the wireless transmission rate calculated for the packetdirected to the first wireless terminal is different from a wirelesstransmission rate calculated for a packet directed to a second wirelessterminal that is in a different state than the first wireless terminal.12. A wireless distribution system comprising: a server; a wireless basestation connected to the server by a network; and at least one or morewireless terminals connected to the wireless base station via a wirelessnetwork using a channel wherein the server includes a first receiverthat receives, from a first wireless terminal, a state of the firstwireless terminal, a priority controller that decides a priority to beassigned to a packet directed to the first wireless terminal based onthe received state of the first wireless terminal, a generator thatgenerates the packet directed to the first wireless terminal, the packetincluding priority data regarding the priority assigned to the packetdirected to the first wireless terminal, an acquirer that acquireschannel-used time that is time over which the channel is used per unittime at the wireless base station, a first calculator that calculateschannel-usable time that is time over which the channel can be used perunit time, for the wireless base station to use the channel to transmitthe packet to the first wireless terminal, based on the channel-usedtime, a second calculator that calculates a channel usage estimated timethat is time estimated to be used for the wireless base station to usethe channel to transmit the packet to the first wireless terminal, andcalculates a wireless transmission rate to be applied when the wirelessbase station transmits the packet to the first wireless terminal, basedon the priority indicated by the priority data included in the packet,to where the channel usage estimated time is within the channel-usabletime, and a first transmitter that transmits the packet, and informationindicating the wireless transmission rate, to the wireless base station,wherein the wireless base station includes a second receiver thatreceives the packet and the information indicating the wirelesstransmission rate from the server, a decider that decides the wirelesstransmission rate based on the information indicating the wirelesstransmission rate, and a second transmitter that transmits the packet bymulticast or broadcast to the first wireless terminal, at the decidedwireless transmission rate, and wherein the wireless transmission ratecalculated for the packet directed to the first wireless terminal isdifferent from a wireless transmission rate calculated for a packetdirected to a second wireless terminal that is in a different state thanthe first wireless terminal.